[86] Computers are
used more extensively on the Space Transportation System (STS)
than on any previous aircraft or spacecraft. In conventional
aircraft, mechanical linkages and cables connect pilot controls,
such as the rudder pedals and stick, to hydraulic actuators at the
control surfaces. However, the Shuttle contains a fully digital
fly-by-wire avionics system. All connections are electrical and
are routed through computers. To give the spacecraft more
autonomy, system management functions (fuel levels, life support,
etc.), handled on the ground during previous flight programs, are
monitored on board. Software can be adjusted to suit increasingly
complex and varied payloads. Subsystems, like the main engines,
that had no computer assistance before use them for performance
improvement. And, as in Gemini and Apollo, guidance and navigation
tasks are accomplished on the Shuttle with computers. All these
functions, especially flight control, are critical to mission
success; therefore, the computers performing the tasks must be
made fail-safe by using redundancy. Meeting these requirements has
resulted in one of the most complex software systems ever produced
and a computer network within the spacecraft with more powerful
hardware than many ground-based computer centers in the
mid-1960s.

The major differences between the Shuttle
computer system and the systems used on Gemini and Apollo were the
choice of an "off-the-shelf" main computer instead of a
custom-made machine and the pervasiveness of the system within the
spacecraft, since the main computers are the heart of any true
avionics system. Avionics (aviation plus electronics) grew in the
1950s and 1960s as electronic devices, especially digital devices,
replaced mechanical or analog equipment in aircraft. These digital
devices were combined into a coherent system, rather than isolated
in function and location within the aircraft. Several modem
military airplanes have applied this concept to varying degrees.
The FB-111, an Air Force tactical bomber, has a complex avionics
system that Rockwell International built just before it was
awarded the Shuttle contract1; the F-15 fighter used an AP-1 computer in its
system. A repackaged version of the F-15's computer became the
AP-101 used in the shuttle2. However, in no aircraft has the Shuttle's avionics
system been matched as yet. For instance, its main computers have
to interconnect with other computers in subsystems, such as the
controllers on each main engine, whereas most aircraft systems are
centered on a single set of machines.

Since the Shuttle is completely dependent
on the success of its avionics system, each component must be made
failure proof. The method chosen to ensure this is absolute
redundancy, often to a depth of four duplicate devices. Managing
this level of redundancy became a large problem in itself.

Another result of the pervasive avionics
system is that the frequency and sophistication of the crew
interaction with the computers exceeds any previous manned space
program. A large portion of the....

[87]

Figure 4-1. The first launch of
the Shuttle Challenger, one of a fleet of the most computationally
intensive spacecraft ever built. (NASA photo)

[88] .....software is
directed at easing the necessary commanding of the computers. In
general, software development for the Shuttle has far outstripped
any previous NASA ground or flight system in effort and cost. The
combination of requirements forced the Agency to pioneer
techniques in digital avionics, redundancy management, computer
interconnection, and real-time software development.